Move developer-hub to polkadot-sdk-docs (#2598)

This PR is a continuation of
https://github.com/paritytech/polkadot-sdk/pull/2102 and part of an
initiative started here https://hackmd.io/@romanp/rJ318ZCEp
What has been done:
- The content under `docs/*` (with the exception of `docs/mermaid`) has
been moved to `docs/contributor/`
- Developer Hub has been renamed to Polkadot SDK Docs, and the crate has
been renamed from `developer-hub` to `polkadot-sdk-docs`
- The content under `developer-hub/*` has been moved to `docs/sdk`

---
Original PR https://github.com/paritytech/polkadot-sdk/pull/2565, it has
been close due to too many rebase conflicts

---------

Co-authored-by: Serban Iorga <serban@parity.io>
Co-authored-by: Chevdor <chevdor@users.noreply.github.com>
Co-authored-by: Egor_P <egor@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

.gitlab/pipeline/test.yml

@@ -313,7 +313,7 @@ node-bench-regression-guard:
   after_script: [""]
 
 # if this fails run `bot update-ui` in the Pull Request or "./scripts/update-ui-tests.sh" locally
-# see ./docs/CONTRIBUTING.md#ui-tests
+# see ./docs/contributor/CONTRIBUTING.md#ui-tests
 test-frame-ui:
   stage: test
   extends: